[UWP]Fixes #3886, Master Detail, incorrect master height after window resize (#5338)

* Fix Issue 3386

* Update Xamarin.Forms.Platform.UAP/MasterDetailControl.cs

Co-Authored-By: bmacombe <brian@agspectrum.com>

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue1461.cs

@@ -228,9 +228,10 @@ namespace Xamarin.Forms.Controls.Issues
 							BackgroundColor = Color.Gray
 						},
 						btn
-					}
 					},
-				//BackgroundColor = Color.Red
+					BackgroundColor = Color.WhiteSmoke
+				},
+				BackgroundColor = Color.Gray
 			};
 
 			if(initState.HasValue)

Xamarin.Forms.Platform.UAP/MasterDetailControl.cs

@@ -161,18 +161,12 @@ namespace Xamarin.Forms.Platform.UWP
 		{
 			get
 			{
-				// Use the ActualHeight of the _masterPresenter to automatically adjust for the Master Title
+				double height = ActualHeight;
-				double height = _masterPresenter?.ActualHeight ?? 0;
-
-				// If there's no content, use the height of the control to make sure the background color expands.
-				if (height == 0)
-					height = ActualHeight;
-
 				double width = 0;
 
 				// On first load, the _commandBar will still occupy space by the time this is called.
 				// Check ShouldShowToolbar to make sure the _commandBar will still be there on render.
-				if (_firstLoad && _commandBar != null && ShouldShowToolbar)
+				if ((_firstLoad || !ShouldShowSplitMode) && _commandBar != null && ShouldShowToolbar)
 				{
 					height -= _commandBar.ActualHeight;
 					_firstLoad = false;